void main(List<String> args) {
  //1.if语句
  int a = 10;
  if (a > 0) {
    print('a是正数');
  }
  //switch语句
  switch (a) {
    case 1:
      print('1');
      break;
    case 2:
      print('2');
      break;
    default:
      print('默认');
  }
  //2.if...else语句
  if (a > 0) {
    print('a是正数');
  } else {
    print('a是负数');
  }
  //3.if...else if...else语句
  if (a > 0) {
    print('a是正数');
  } else if (a < 0) {
    print('a是负数');
  } else {
    print('a是0');
  }
  //4.if语句嵌套
  if (a > 0) {
    if (a > 5) {
      print('a是大于5的正数');
    } else {
      print('a是小于等于5的正数');
    }
  } else {
    print('a是负数');
  }




}

